Passive, high-temperature amplifier for amplifying spark signals detected in igniter in gas turbine engine

ABSTRACT

A system for detecting spark in an igniter for a gas turbine engine. An igniter generates a plasma, or spark, somewhat similar to an automotive spark plug. In the invention, an inductive pick-up is positioned adjacent the igniter, to detect current pulses in the igniter, to thereby infer the presence of spark. The signal produced is small, and requires amplification. However, the environment is hot, often exceeding 400 degrees F. The invention utilizes an amplifier composed of passive components, in the form of an RLC circuit.

CROSS REFERENCE TO RELATED APPLICATIONS

This Application is related to subject matter in the following patentapplications, which are of common inventorship and filed concurrentlyherewith:

SENSOR FOR DETECTION OF SPARK IN IGNITER IN GAS TURBINE ENGINE, Ser. No.______;

METHOD OF INFORMING PILOT OF AIRCRAFT OF SPARK DETECTED IN GAS TURBINEENGINE, Ser. No. ______;

INTEGRAL SPARK DETECTOR IN FITTING WHICH SUPPORTS IGNITER IN GAS TURBINEENGINE, Ser. No. ______;

DETECTING SPARK IN IGNITER OF GAS TURBINE ENGINE BY DETECTING SIGNALS INGROUNDED RF SHIELDING, Ser. No. ______; and

SPARK IGNITER FOR GAS TURBINE ENGINE, Ser. No. ______.

FIELD OF THE INVENTION

The invention relates to gas turbine engines, and igniters therein.

BACKGROUND OF THE INVENTION

This Background will explain why the lack of absolute certainty inlifetimes of igniters used in gas turbine aircraft engines can imposesignificant costs on the owners of the aircraft utilizing the engines.

FIG. 1 is a highly schematic illustration of a gas turbine engine 3,containing a combustor 6. Fuel 9 is sprayed into the combustor. Anigniter 12, which functions in a roughly analogous manner to a sparkplug in an automobile, produces a spark, or plasma discharge (notshown), which initially ignites the jet fuel.

After initial ignition, the igniter 12 can be repeatedly sparkedthereafter, primarily as a safety measure. That is, in a modern engine,under normal circumstances, it is extremely unlikely for a flame-out tooccur in the combustor 6. However, unexpected situations, such as anabrupt cross-wind, can affect the environment within the combustor, andresulting loss of flame.

In addition, certain flight conditions make the unlikely event of aflame-out slightly more probable. Thus, for example, the igniter 12 maybe activated when the aircraft enters a rain squall, or other situationwhich may disturb steady-state conditions in the combustor 6.

The igniters 12, like all mechanical components, have useful lives whicheventually expire, at which time the igniters must be replaced. However,this expiration-and-replacement can create a situation in aircraft whichis expensive.

A primary reason is that the approach of an igniter to the end of itslifetime is not marked by readily detectable events. That is, at somepoint, the igniter completely ceases to generate a plasma, or spark.However, prior to that point, the igniter may sporadically generatesparks.

As explained above, the sparking is not, in general, required tomaintain the combustor flame. Consequently, the sporadic sparking wouldonly be noticed if an actual flame-out occurred, and if the sporadicsparking were ineffective to induce a re-light. Since such a combinationof events is seen as unlikely, the sporadic sparking is not readilynoticed. The impending expiration of the useful life of the igniter issimilarly not noticed.

Another reason is that, while all igniters may be constructed asidentically as possible, nevertheless, those igniters do not all possessthe same lifetimes. Nor do all igniters experience identical eventsduring their lifetimes. Thus, it is not known exactly when a givenigniter will expire.

Thus, the point in time when an igniter must be replaced is not knownwith certainty. One approach to solving this problem is to performpreventative maintenance, by replacing the igniters when they are stillfunctioning. While the cost of a new igniter and the manpower requiredto install it is not great, the early replacement does impose anothercost, which can be significant.

The aircraft in which the igniter is being replaced represents a revenuesource measured in thousands of dollars per hour. If the aircraft isrendered nor-functional for, say, two hours during replacement of anigniter, the revenue lost during that time is substantial.

Therefore, the uncertain lifetimes of igniters in gas turbine aircraftengines can impose significant losses in revenue.

SUMMARY OF THE INVENTION

Normal operation of an igniter in a gas turbine engine causes erosion ofan insulator inside the igniter. In one form of the invention, anauxiliary ground electrode is embedded within that insulator, and theerosion eventually exposes the auxiliary electrode. The igniter isdesigned so that the exposure occurs at the time when the igniter shouldbe replaced.

The exposed auxiliary ground electrode can be detected by the fact that,when a spark occurs, a small current travels through the auxiliaryground electrode. When that current is detected, its presence indicatesthe exposure. Alternately, the exposed auxiliary ground electrode can bevisually detected by a human observer, perhaps by using a borescope.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a simplified schematic of a gas turbine engine.

FIG. 2 illustrates an igniter 12, shown in FIG. 1.

FIGS. 3 and 4 are enlarged views of end E in FIG. 2.

FIGS. 5 and 6 illustrate changes in geometry of end E which theInventors have observed.

FIG. 7 illustrates one form of the invention.

FIGS. 8 and 9 are views resembling insert 84 in FIG. 7.

FIG. 10 is a perspective view of part of FIG. 7.

FIG. 11 is a perspective, cut-away view of one form of the invention.

FIG. 12 is a cross-sectional view of the apparatus of FIG. 11.

FIG. 13 is a perspective view of the apparatus of FIG. 11.

FIG. 14 illustrates one form of the invention.

FIG. 15 illustrates a sequence of events occurring in one form of theinvention.

FIG. 16 illustrates two distances D9 and D10, over which two electricfields are generated.

FIG. 17 illustrates one mode of constructing auxiliary electrode 72 inFIG. 15.

FIG. 18 illustrates an aircraft which utilizes one form of theinvention.

FIG. 19 illustrates igniter 308, bearing an annular coil 310.

FIG. 19A illustrates one form of the invention, wherein a bracket 311supports the igniter, and also contains a coil 310.

FIG. 20 illustrates an igniter-cable assembly about which is positioneda high-permeability ring 326, about which is wrapped a coil 320.

FIG. 21 is a schematic of one view of an igniter system.

FIG. 22 is a schematic developed by the Inventors of a possible mode ofoperation of the apparatus of FIG. 21.

FIG. 23 illustrates a prior-art RLC circuit.

FIG. 24 illustrates an RLC circuit excited by a sinusoidal waveform.

FIGS. 25-28 are plots of simulated output of the circuit of FIG. 24.

FIG. 29 illustrates an RLC circuit pulsed by a pulse train.

FIGS. 30-34 are plots of simulated output of the circuit of FIG. 29.

FIG. 35 illustrates timing parameters.

FIG. 36 illustrates two forms of the invention.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 2 illustrates an igniter 12 used in the prior art. An electricalconnector (not shown) is threaded onto threads 21, and contains anelectrical contact (not shown) which mates with the end 24 of electrode27. Insulator 30 isolates electrode 27 from the shell 33 of the igniter12.

End E of the igniter 12 is shown in FIGS. 3 and 4. A very simplifiedexplanation of the physics involved in plasma generation will be given.

In operation, a high voltage is applied to the electrode 27, therebycreating a voltage difference, or potential difference, V between pointsP1 and P2 in FIG. 3. The electric field in that region equals thepotential difference V divided by the distance D between the points P1and P2. For example, if the voltage is 20,000 volts and the distance Dis 10 millimeters, or 0.01 meter, then the electric field equals20,000/0.01, or 2 million volts per meter.

The electric field is designed to exceed the dielectric breakdownstrength of the material, or medium, lying between points P1 and P2.That material is a mixture of air plus fuel. However, the field does notexceed the breakdown strength of insulator 30, and that strength exceedsthat of the air-fuel mixture.

When breakdown occurs, the electric field strips electrons away from theatoms in the medium, producing positively charged ions and freeelectrons. The electric field drives the free electrons in a directionparallel with the electric field. However, during that movement, thosetemporarily free electrons will collide with other ions. Also, thermalmotion of the ions and electrons will also bring them together incollisions.

In the collisions, the electrons will be captured by the ions, and willdrop to a lower energy state, releasing heat and light, in the form ofan electric arc which is called a plasma, which is indicated aslightning bolt 40 in FIG. 4. This process continues as long as theelectric field is present.

The Inventors have observed one result of the operation just described.As indicated in FIG. 5, the insulator 30 becomes eroded from the phantomshape 50 to the curved shape 53. In addition, the electrode 27 becomeseroded from the phantom shape 56 to the solid shape 59. Corners 33A alsobecome eroded.

The Inventors believe that one or more of the following agencies areresponsible for the erosion. One agency is the corrosive nature of theplasma: free electrons are very reactive, and seek to bind to anyavailable atoms or ions which are nearby. Also, the generation of freeelectrons from oxygen, which is present in the air, creates ionizedoxygen, which is also highly reactive.

A third agency is that the plasma creates a high-temperatureenvironment. A high temperature, by definition, represents agitatedatoms and molecules with high velocities. High-velocity atoms andmolecules react more readily with stationary objects when they collidewith the objects.

Possibly a fourth agency is the fact that the plasma generateshigh-frequency photons, in the ultra-violet, UV, and perhaps into theX-ray regions of the spectrum. It is well known that UV and X-radiationcan damage numerous types of material.

Irrespective of the precise causes of the erosion, the erosionillustrated in FIG. 5 eventually causes the igniter 12 to eventuallystop functioning. A primary reason is illustrated in FIG. 6. Previously,prior to the erosion, voltage was applied between points P1 and P2 inFIG. 6. However, after the erosion, point P2 has effectively moved topoint P3. Distance D has now become longer distance D2. The electricfield, which causes the ionization and thus the plasma, is now weaker.

Continuing the example given above, if distance D2 is 20 millimeters,then the electric field becomes 20,000/0.020, or one million volts permeter, half its original value. Eventually, distance D2 becomes so greatthat the electric field does not reliably exceed the dielectricbreakdown strength of the air-fuel mixture, and ionization ceases tooccur.

FIG. 7 illustrates one form of the invention. An auxiliary electrode 72is embedded in the insulator 75. The tip 78 is covered by theinsulator-material in region 81, as indicated by the insert 84.Auxiliary electrode 72 may be connected to the shell 33, as at region90.

Initially, current enters electrode 27 as indicated by arrow 84, jumpsto the shell 33 through the plasma 85, and exits the shell 33 into theengine, through multiple paths, such as through its mounting threads, asindicated by arrow 86.

As erosion occurs, the insulator 75 departs from its initial shapeindicated by phantom lines 92 in FIG. 8. Tip 78 of the auxiliaryelectrode 72 now becomes exposed. Now, when a high voltage is applied tothe igniter, two paths exist for a plasma to follow. One is the usualpath P5 in FIG. 9. The other path is indicated as P6 of FIG. 9, and runsfrom the central electrode 27 to the now-exposed auxiliary electrode 72.

Restated, two current-return-paths are available to the centralelectrode 72. Path P5 runs to the shell 33, in the usual manner. Path P6runs to the now-exposed auxiliary electrode 72. Eventually, furthererosion will lengthen path PS, and cause plasma formation along thatpath to terminate. That is, path PS in FIG. 9 initially can berepresented by distance D in FIG. 6. After sufficient erosion, path PSin FIG. 9 will be represented by distance D2 in FIG. 6, and, asexplained above, no plasma will be generated along path PS when distanceD2 becomes sufficiently large.

However, auxiliary plasma path P6 is still available in FIG. 9 at thistime. A plasma can still be generated, and the lifetime of the igniterhas been increased.

The preceding discussion presented the auxiliary electrode 72 in FIG. 7in the form of a rod. FIG. 10 illustrates such a rod in perspectiveview, surrounded by insulator 75.

In an alternate embodiment, a cylinder is used. FIG. 11 is a cut-awayview of one embodiment. Central electrode 27 is surrounded by aninsulator 100, which itself is surrounded by a conductive tube orcylinder 103, which is then surrounded by another layer of insulator105. FIG. 12 illustrates the system in cross-sectional view, withsimilar numbering.

FIG. 13 illustrates the insulator 100 in its initial configuration,after manufacture or just after installation. A tip 110 of centralelectrode 27 is exposed, and surrounded by the conical surface 113 ofthe insulator 100. Cylindrical auxiliary electrode 103 is embeddedwithin the insulator 100, and no tip or edge is exposed, as indicated bydistance D8 in FIG. 12.

The preceding discussion stated that the auxiliary electrode 72 may beconnected at region 90 in FIG. 7. In another embodiment, the auxiliaryelectrode 72 of FIG. 14 is also connected to ground, but through adetector 150. Detector 150 looks for a current in auxiliary electrode72. Current detectors are well known.

If no current is detected, it is inferred that the auxiliary electrode72 is still embedded within insulator 75, as in FIG. 7, and iselectrically isolated from central electrode 27.

In contrast, if a current is detected, it is inferred that the auxiliaryelectrode has become exposed through erosion, as in FIG. 9. The detectedcurrent is attributed to a plasma following path P6. When the current isdetected, detector 150 issues a signal, sets a flag, or otherwiseindicates the inference that erosion has exposed auxiliary electrode. Ahuman technician at that time, or a prescribed time afterward, replacesthe igniter.

An alternate mode of detection is to remove the igniter and visuallyexamine the end corresponding to end E in FIG. 2. If a smooth surface ofthe insulator 100 is seen, as in FIG. 13, then it is concluded that theigniter is still functional. However, if the auxiliary electrode 72 isseen, as in FIG. 8, then it is concluded that replacement may berequired.

In another embodiment, the auxiliary electrode is designed to becomeexposed, and then to erode rapidly. FIG. 15, viewed left-to-right,illustrates first a newly installed igniter 160. After a period ofusage, igniter 165 exposes its auxiliary electrode 72. Now a plasma P6extends to the auxiliary electrode 72.

However, as stated above, the auxiliary electrode 72 is designed toerode rapidly. For example, as insert 170 indicates, the auxiliaryelectrode 72 is fabricated with a pointed end. Plasma 6 causes thepointed end to become rapidly eroded, as indicated by the smallparticles in frame 170. This operation causes a specific sequence of twoevents.

One is that, when the auxiliary electrode becomes first exposed, acurrent passes through the it. The current is detected, as by detector150 in FIG. 14. Next, after the auxiliary electrode fractures or erodes,no current passes through it.

One reason for this sequence is illustrated in FIG. 16. Initially, thevoltage V spans distance D9, creating an electric field equal to V/D9.After fracture or erosion, the same voltage V spans distance D10. Theelectric field equals V/D10, a smaller value. The latter electric fieldis insufficient to create a plasma, while the former is.

In one embodiment, the occurrence of the two events just describedoccurs prior to the termination of the lifetime of the igniter. Thus,that termination is signalled by the occurrence of a current through theauxiliary electrode 72, followed by a termination of that current. Theonset of the current indicates the approach of the termination of thelifetime, but with time remaining to operate the engine. The subsequenttermination of the current indicates that less time remains, and thatreplacement of the igniter becomes more important.

FIG. 17 illustrates one embodiment of the auxiliary electrode 72. Aneck, or groove, 190 is provided, which facilitates the breakageschematically illustrated in the insert 170 in FIG. 15. The groove 190is a region of mechanical weakness intentionally built into theauxiliary electrode 72. Prior to the erosion indicated in FIG. 8, thatweakness is not important, because mechanical support to the electrodeis supplied by the insulator 75.

The discussion above stated that a high voltage is applied to electrode27. It is possible that a low voltage applied to the electrode 27 canaccomplish the same function of generating a plasma.

FIG. 18 illustrates another embodiment. An aircraft 300 is powered bygas turbine engines (not shown), which are located within nacelles 305.Each engine contains one, or more, igniters, as discussed above. Theigniters may contain an auxiliary electrode, as described above, or maybe of the prior-art type.

FIG. 19 illustrates an igniter 308. The invention adds a sensor, such assensing coil 310. This coil 310 is coaxial with the igniter, asindicated. This particular coaxial arrangement was used in anexperiment, to ascertain whether a coil could detect a signal when theigniter 308 produced a spark.

The coaxial arrangement is not necessarily required. In one form of theinvention, coil 320 can be arranged as shown in the two rightmost imagesin FIG. 20. The magnetic field lines B produced by current I in thepower cable 315 are concentric with the current which produces the fieldlines B. Since that current flows though the entire cable 315, andthrough the igniter 308, the field lines B extend along both the cable315 and the igniter 308.

According to Faraday's Law, optimal coupling is attained when the coil320 is perpendicular to the field lines B, as shown in the centralimage.

In another form of the invention, a high-permeability ring 326,constructed perhaps of transformer iron, is placed about the igniter, orcable 315, and the coil 320 is wrapped about the ring 326. The ring 326captures the magnetic field lines B, as it were, and delivers them tothe coil 320. Under this arrangement, field lines B passing through thering 326 also pass through the coil 320.

One definition of the term high permeability is that the relativepermeability of a high permeability material exceeds 1,000. As a pointof reference, the relative permeability of many steels is in the rangeof 4,000. Materials exist having relative permeabilities approaching onemillion.

In yet another form of the invention, a prior-art clamp-on currentdetector (not shown) is used.

The Inventors have found that the coil 310 in FIG. 19, even thoughlacking the perpendicular characteristic shown in FIG. 20, produced adetectable signal in response to current pulses within the igniter 308.The Inventors offer the following observations concerning this signal,and its detection.

FIG. 21 is an electrical schematic of the igniter circuit. Block 330represents the exciter, which is contained within a conductive housing(not shown). The exciter 330 produces a high-voltage pulse train, togenerate spark in the igniter 340.

Power cable 335 delivers the high-voltage current pulses to the igniter340. One type of high-voltage pulses are in the range of 20,000 volts.One type of frequency of the pulses lies in the range of 10 Hz, that is,ten pulses per second. One type of pulse has a duration of 10milliseconds. In this example, the duty cycle is thus ten percent(0.10/0.100).

Plot 331 illustrates the pulses just described. Duration D would be 10milliseconds in this example. Period T would be 100 milliseconds in thisexample, corresponding to a frequency of 1/T, or 10 pulses per second.

In FIG. 21, a shield 345 surrounds the power cable 335. The shield 345can take the form of solid conduit, a woven conductive sleeve, acombination of the two, or other types of shielding. This shield 345 isconnected to the casing 350 of the igniter 340, and the casing 350 isconnected to the frame, or housing, 352 of the engine, which isconsidered to be a DC ground.

The shield 345 provides electromagnetic interference suppression andalso prevents personnel from contacting the high-voltage cable 335. Eventhough the cable 335 is itself surrounded by a thick insulating cover,the shield 345 provides a redundant safety measure.

Under the arrangement illustrated, the housing (not shown) of theexciter 330, the shield 345, and the casing 350 of the igniter 340 areall connected to the frame 352 of the engine, and are considered to beheld at DC ground.

One type of classical analysis of the apparatus shown in FIG. 21 statesthat the following mechanism can explain the sparking operation. Duringeach high-voltage pulse, of duration D in plot 331, the current providedby the high-voltage cable 335 reaches the spark gap 355, jumps the gap355, and returns via the frame of the engine, to the exciter 330, alongpath 360. Under this mechanism, arguments can be mustered indicatingthat the coil 310 of the type shown in FIG. 19 would be ineffective todetect the current pulses.

A basic argument is that, in theory, coil 310 should detect no current,because no magnetic field lines B, in theory, penetrate thecross-sectional area of the coil 310. Faraday's Law states that suchpenetration is required.

In considering additional arguments, two cases should be distinguished:the DC case and the AC case. In the DC case, if a DC current is carriedby the high-voltage cable 335 in FIG. 21, then static magnetic fieldlines B of the type shown in FIG. 20 would be present. If the coil isplaced around the igniter as shown in FIG. 19, and if the casing 350 ofthe igniter is constructed of a high-permeability material, such as atype of steel, the casing will trap some, or all, of the B-field, andpossibly inhibit detection of the static B-field. It could be said thatthe shielding, including the casing 350 of the igniter and the shield345, act as a Faraday cage which contains the static DC magnetic field.Thus, an argument may exist stating that coil 310 in FIG. 19 would notdetect the current pulses.

This argument may also apply to slowly varying currents. That is, it ispossible that the Faraday cage also blocks slowly changing magneticfields.

In the AC case, electromagnetic radiation may emanate from the cable 335and igniter 350 in FIG. 21, particularly because the current pulsescontain high-frequency components. High-frequency components, ingeneral, radiate more readily, at least from short antennas of lengthequal to fractions of a wavelength. The conductivity, as opposed topermeability, of the shield 345 and casing 350 can block radiationproduced by the current pulses. In one mechanism, the blockage occursthrough reflection: the radiated electromagnetic field induces currentsin the shield 345 and casing 350, which radiate their ownelectromagnetic fields inwardly, toward the cable 335. The radiatedfields, in effect, reflect the incoming radiation back to the cable 335.

Also, as stated above, the shield 345 and igniter casing 350 areconnected to ground. In theory, those grounded elements shunt all ACsignals to ground, and thus prevent them from radiating electromagneticenergy.

Thus, at least the preceding arguments exist which indicate that thecoil of FIG. 19 would not be effective to detect current pulses in thecable 335 of FIG. 21.

The Inventors have observed, or postulated, that all the return currentmay not follow path 360 in FIG. 21. Return current refers to thatreturning to the exciter 330 after having jumped the spark gap 355. TheInventors surmised that some return current may travel along theshielding system, including the shield 345 and the casing 350 in FIG.21. FIG. 22 is one representation of this surmise.

Resistor R1 indicates the small resistance of the ground path throughthe engine frame, from the spark gap 355 to the exciter, and correspondsroughly to path 360 in FIG. 21. Resistor R2 indicates the smallresistance of the path from the spark gap 355 to the exciter, butthrough the shield system. The shield system includes the casing 350 ofthe igniter and the shield 345. Both R1 and R2 originate at, or near,the spark gap 355, but represent different routes to the exciter 330.

Under this surmise, it is possible that the return current passingthrough R2 is detectable, despite the arguments given above.

In this context, the Inventors point out that, in general, detecting areturn current through the other resistor R1 is considered impractical.Resistor R1 represents, among other things, the engine itself. A simple,accurate, and reliable approach to detecting return current in theengine is not seen as practical, at least for the reason that the numberof paths available to the return current is so large, in spanning theentire engine, or a large part of it.

However, the path represented by resistor R2 is a localized, discreteentity, from a current-detection standpoint, and is not contained withina shield. Thus, if a return current pulse is travelling in R2, then themagnetic field, or electromagnetic radiation, produced by the pulse maybe detectable, by detecting the current in R2, that is, the current inthe shield system.

An experiment was undertaken, using coil 310 in FIG. 19, and it wasfound that coil 310 produced a detectable signal, when the exciterproduced a spark pulse.

It should be observed that the coil 310 may be detecting one, or more,of the following currents. Coil 310 may be detecting the current pulsein the cable 335 in FIG. 21, contrary to the arguments given above: theshielding system may not be completely effective. Alternately, coil 310in FIG. 19 may be detecting return current pulses in the casing 350 ofthe igniter, as postulated above. Or coil 310 may be detecting some typeof sum or difference of the two currents just identified.

The signal detected in coil 310 was small, so that amplification may bedesired. However, the operational environment of the coil 310 provides achallenge in this respect.

During operation of the aircraft 300 of FIG. 18, coil 310 in FIG. 19will be located in an environment having a temperature exceeding 400 F.That is, the casing 350 of the igniter 308 in FIG. 19, to which the coil310 is attached or adjacent, exhibits a temperature of at least 400 Fduring normal operation.

Such a high temperature would cause problems if a solid-state amplifieris to be used to amplify signals produced by coil 310. Nevertheless,with sufficient precautions, an electronic, transistorized amplifiercould be used to detect the signals produced by the coil 310.

In one form of the invention, no solid-state amplifier is used, at leastnot in the vicinity of the 400 degree environment. Instead, a passiveamplifier was developed, using only resistive, capacitive, and inductiveelements, with no active elements such as transistors or vacuum tubes.One definition of active element is that an active element can amplifypower of an input signal: output power can exceed input power. A passiveelement does not possess that property of power amplification.

It is known that a series RLC circuit, such as that in FIG. 23, can bedesigned to produce an amplified voltage across the capacitor C undercertain conditions. This amplification is discussed in Chapter 13,entitled “Frequency Response,” in the text entitled “Engineering CircuitAnalysis,” by William Hayt and Jack Kemmerly (ISBN 0-07-027410-X,McGraw-Hill, 1993). This text is hereby incorporated by reference.

The conditions for amplification include the following. One, the signalsource, Vin, be sinusoidal, and of constant frequency, which is asituation often called sinusoidal steady-state. Two, the values of thecapacitor C and inductor L are chosen so that the input impedance seenat points P1 and P2 is purely real, with no reactive components. Thiscondition is called resonance, and the value of the resonance frequency,omega-sub-zero, is indicated in FIG. 23.

Under these conditions, the voltage across the capacitor, Vc, will equalABS(Q)×Vin, as indicated in FIG. 23, wherein ABS refers to the absolutevalue, or magnitude, of the Quality factor Q of the circuit. Q isdefined as indicated in FIG. 23. Thus, for example, for a Q of 10, atenfold amplification is attained.

Thus, the prior art indicates that a series RLC circuit can providesvoltage amplification to a sine wave input. The text identified aboveindicates that the dual of a series RLC circuit, namely, a parallel RLCcircuit, provides current amplification, as opposed to voltageamplification.

A computer simulation will illustrate the voltage amplification.

FIG. 24 illustrates a circuit which was simulated using one of thecommercially available SPICE programs. The transformer 372 is present,in order to make this circuit consistent with the circuit models of theinvention, later discussed. Values of the resistor R, capacitor C, andinductor L are indicated. These values remained constant in allsimulations.

FIGS. 25-28 illustrate results of four simulations done on the circuitof FIG. 24. An input signal, Iin in FIG. 24, was applied, in the form ofa two-amp peak-to-peak sinusoid, which is shown in the Figures. Thefrequency of the input signal was changed in each simulation.

In FIG. 25, the input frequency was 50 Hz. The left axis applies to theinput signal Iin. The right axis applies to the voltage across thecapacitor, Vc, which is indicated in FIG. 25. It is clear that, at 50Hz, the output Vc is a sinusoid of about 200 volts, peak-to-peak.

In FIG. 26, the input frequency was 750 Hz. It is clear that, at 750 Hzin FIG. 26, output Vc is a sinusoid of about 3,200 volts, peak-to-peak.

The resonance frequency of FIG. 24 is about 2517 Hz, computed using theexpression for omega-sub-zero given in FIG. 23. In FIG. 27, the inputfrequency was 2517 Hz. It is clear that, at 2517 Hz, Vc is a sinusoid ofabout 32,000 volts, peak-to-peak.

In FIG. 28, the input frequency was 50 kHz, that is, 50,000 Hz. It isclear that, at 50 kHz, Vc is a sinusoid of about 200 volts,peak-to-peak.

FIGS. 25-28 are consistent with the postulate that a series RLC circuitcan amplify a steady-state sinusoid. At the resonant frequency, Vc ishigh, 32,000 volts at resonance, and at other frequencies, Vc is lower.It is emphasized that FIGS. 25-28 do not represent voltages produced bycoil 310 in FIG. 19, but capacitor C in FIG. 24, under the assumedconditions.

The Inventors investigated whether an RLC circuit of the type shown inFIG. 24 can produce a similar amplification when the input signal is nota steady-state sinusoid, but a train of pulses of the type used to powerthe igniters discussed herein. Experimental results indicate anaffirmative answer, and the computer simulations to be discussed provideplausibility arguments.

In FIG. 29, coil 370 represents the power cable 315. The power cable isactually a single-turn device, but coil 370 in FIG. 24 is represented asa multi-turn device, in order to emphasize the usage of the power cableas the primary of a transformer 373.

Coil 375 represents the pick-up coil 310 of FIG. 19, but coilsresembling coil 320 in FIG. 20 could be used. In FIG. 29, capacitor Cand resistor R are elements added to the sensing coil 375, in thepursuit of amplification. It is emphasized that the elements of thecircuit of FIG. 29 are chosen to withstand operating temperaturesconsistent with the environment in which they will be used, particularlytemperatures, and also vibration.

The Inventors have found that, for a given pulse train, an artificialresonance frequency can be first computed. Then, in one approach, theartificial resonance frequency is treated as an ordinary sinusoidalsteady-state resonance frequency, corresponding to omega-sub-zero inFIG. 23. Using the artificial resonance frequency, values of inductor Land capacitor C are chosen in the usual manner, but recognizing that (1)an artificial resonance frequency is being used and (2) steady-statesinusoidal resonance will not apply. Instead, the L and C valuesobtained are used with a pulsed input.

In practice, the value of inductor L may be fixed by the materials andgeometry of used to construct coil 310 in FIG. 19, so that the onlyvalue under control of the designer would be that of capacitor C.

Once the values of L and C are chosen, based on the artificial resonancefrequency, it is found that amplification of a pulse train applied totransformer 372 in FIG. 29 can occur.

Alternately, the artificial resonance frequency can be determinedgraphically, and this will be illustrated by a sequence of examples.FIG. 30 illustrates simulation output of the circuit of FIG. 29, butwhen excited by the input signal 400 in FIG. 30, which is a triangularcurrent pulse, which is applied to coil 370 in FIG. 24. The componentvalues used in FIG. 25 for the current simulation were the following: Rof 500 ohms, L of one Henry, and C of 0.40 microFarads, as indicated inFIG. 30.

The horizontal axis indicates time, in units of milliseconds. As before,the left axis applies to the input signal, and the right axis applies tothe output signal, which is the voltage across capacitor C in FIG. 24.

FIG. 30 indicates that the output is a decaying sinusoid which firstpeaks at about positive 250 volts, at point 405, then peaks at aboutnegative 175 volts, at point 410, and so on. This output response iscommonly called an underdamped response in an RLC circuit, and is alsocalled ringing.

FIG. 31 illustrates a simulation using the same triangular input, andthe same component values as in FIG. 30, with the exception thatcapacitor C is one-tenth its previous value, and is now 0.040microFarads. It is seen that the response frequency increases,consistent with the reduction of the value of C. Further, the amplitudeof Vc has increased: it now peaks at about 2.4 kvolts, at point 415.

FIG. 32 illustrates a simulation using the same triangular input, andthe same component values as in FIG. 31, with the exception thatinductor L has been cut in half, and is now 0.5 Henry. It is seen thatthe response frequency increases, consistent with the reduction of thevalue of L. Further, the amplitude of Vc has increased: it now peaks atabout 2.8 kvolts, at point 420.

FIG. 33 illustrates a simulation using the same triangular input, andthe same component values as in FIG. 32, with the exception thatinductor L has been cut to 20 percent of its previous value, and is now0.1 Henry. It is seen that the response frequency increases, consistentwith the reduction of the value of L. Further, the amplitude of Vc hasincreased: it now peaks at about 3.3 kvolts, at point 425.

FIG. 34 is an expanded view of FIG. 33, spanning from zero to 2.0milliseconds, and illustrates one concept of the artificial resonancefrequency. Time T illustrates the period of the response. The responsefrequency F, in Hz, is of course 1/T. F is the resonance frequency ofthe circuit, and is defined in FIG. 24.

FIGS. 30-34 illustrate a graphical approach to selecting an artificialresonance frequency. In a sense, one selects values of L and C to attainan output waveform, as in FIG. 34, wherein the upper half of the firstsine wave resembles the input waveform. For example, upper half 425somewhat resembles the input wave form. Alternately, a more mathematicalapproach can be used.

In FIG. 34, the value T/2 can be termed the half-period of the circuit'sresonance frequency. T/2 is the duration of one positive-going, or onenegative-going, hump of the decaying sine wave. The Inventors point outthat, in the sequence of FIGS. 30-34, as T/2 of FIG. 34 approaches theduration TT in FIG. 34 of the triangular input pulse, voltageamplification increases. That is, as T/2 approaches duration TT, bychanging the values of L and C in FIG. 24, capacitor voltage Vcincreases.

This illustrates one approach to setting the artificial resonancefrequency. Inductor L and capacitor C are selected so that they producea resonance frequency having a half-period T/2 equal to the duration TTof the input pulse. But the input is pulsed, or in the case of thesimulations of FIGS. 30-34, triangular.

This computation can also be used if the input pulse is a rectangularpulse, and not the triangular pulse shown. The duration of the inputpulse, such as D in plot 331 in FIG. 21, is taken as corresponding toduration TT in FIG. 34. Duration D then corresponds to a frequency of1/D, in Hz. That frequency is taken as the artificial resonancefrequency, and the needed L and C are computed, using the equationsgiven in FIG. 23, with suitable conversion to radian measure.

In another approach, the artificial resonance frequency is chosen basedon the rise time, or fall time, of the input pulse. In FIG. 34, risetime is about one microsecond, and is the time to rise from point 430 topoint 435. The artificial resonance frequency is chosen so that T/4,that is, one-fourth of a period T, equals the rise time. Values for Land C are chosen accordingly.

A similar principle applies to the fall time.

The input pulse may be a rectangular pulse. Of course, the pulse willnot be perfectly rectangular: the leading and trailing edges willnecessarily have finite rise and fall times. The artificial resonancefrequency is chosen so that T/4, namely, the quarter-period time, equalsthe rise time, analogous to the triangular case.

It is pointed out that the artificial resonance frequency is largelydetermined by the period D in FIG. 1. However, period D is not afrequency of the pulse train. Rather, the frequency equals 1/T. Thus,the artificial resonance frequency, and thus the values of L and Ccomputed from that frequency, do not depend completely on the inputfrequency, but also on the duty cycle of the pulse train.

The preceding approaches discussed selecting an artificial resonancefrequency, based on timing of input pulses. In yet another form of theinvention, the artificial resonance frequency is determined bytrial-and-error. Simulations are run, as by experimenting with theactual circuit shown in FIG. 24, or by using computer software such asSPICE, which simulate that circuit. Various values of L and C areselected, and those providing the desired amplification are then used.

However, the sizes of L and C can be limited by practicalconsiderations. For example, in a given situation, attainment ofresonance may require a capacitor which is extremely large in physicalsize. Thus, in some situations, components can be chosen to provideoperation at a non-resonant condition, but still provide adequateamplification.

It is pointed out that the computer simulation approach can be verysimple, given that many SPICE programs allow parameters to be swept.That is, in sweeping, a range of values for a parameter, such as L, isselected, and the number of values to be used in the range is specified.In sweeping, literally thousands or millions of different values of L,and also C, can be selected and tested, all using a computer program,with little or no human effort.

A human then examines the results and chooses those desired.

At the artificial frequency, values of L and C are computed whichprovide specific impedances. Those impedances are the phasor impedancescomputed as if the excitation were steady-state sinusoidal. That is, atthe artificial resonant frequency, the sum of the impedances of L and Care set to zero.That is, jwL+1/jwC=0, whereinL is the inductance,C is the capacitance,w is the pseudo-resonant frequency, andj is the imaginary operator.Once w is chosen, L and C are selected to satisfy the equation given inthe preceding sentence.

Thus, the desired values of L and C are computed as though the systemwere operating in a steady-state sinusoidal mode, but then L and C areused in a pulsed input mode.

In addition, the value of R can be important. In one form of theinvention, the RLC circuit of FIG. 24 is designed to exhibit anunderdamped response, in the engineering sense, so that excitation by apulse will induce the sinusoidal response called ringing. The envelopeof the sinusoid decays exponentially. The value of R determines thespeed of decay. In one form of the invention, R is chosen so that thefollowing events occur.

First, a current pulse generates a spark. That pulse excites the RLCcircuit, such as that shown in FIG. 29. The RLC circuit goes intoringing, as indicated in FIGS. 31-34. But the R is selected so that theringing sinusoid decays sufficiently before the next pulse, so that thenext ringing sinusoid can be distinguished from the current one.

In one embodiment, the ringing sinusoid in FIG. 35 decreases to 50percent of its original amplitude A within ½ T. Also contemplated is adecrease to 50 percent within any selected time between 0.05 and 0.9 T.

U.S. Pat. No. 5,523,691, Jun. 4, 1996, Application Number 458,091,issued to Frus, illustrates one approach to detecting spark in anigniter in an aircraft engine. Frus states that inductor L1 in his FIG.1A is first charged by a current. When a normal spark occurs, theinductor L1 rapidly discharges through the IGNITER PLUG.

However, if the IGNITER PLUG does not produce a spark, then the inductorL1 sees the IGNITER PLUG as a very high resistance. In this case,inductor L1 discharges through a resistor contained within voltagedivider 27. This latter discharge requires a significantly longer time.Frus detects the length of the discharge, and when the longer dischargeis detected, he infers the absence of spark.

Frus also states that spark can fail to occur if the voltage produced byexciter 13 falls short of the intended value. Frus discusses an approachto detecting this failure.

The Inventors point out that the inductor L1 in Frus does not serve asimilar purpose to the coil 310 in FIG. 19 herein. For example, Frus'inductor L1 carries current which is delivered to the IGNITER PLUG. Incontrast, coil 310 does not do that.

Another difference is that Frus' inductor L1 must be designed towithstand voltages which certainly exceed 1,000 volts, and probablyexceed 20,000 volts. Thus, significant insulation is required betweenthe input and output leads extending from the physical inductor L1, aswell as around the coiled wire within the device. In contrast, coil 310of FIG. 19 must withstand a few volts. In one embodiment, coil 310 isdesigned as an inductor with an operating voltage on its two leads of nomore than 5, 10, or 100 volts, in three different embodiments.

In addition, the inductor L1 of Frus does not seem to be present in anenvironment where the temperature exceeds 400 F.

In one form of the invention, a specific starting sequence is used whenthe gas turbine engine is started, as in the aircraft shown in FIG. 18.The pilot causes a starting system to rotate the rotor (not shown) ofthe engine, or orders a control system (not shown) to initiate astart-up routine. A fuel control (not shown) delivers fuel to thecombustor. The igniters in the combustors are actuated.

If light-off of the engine is not detected, the pilot then examines anindicator 500 in FIG. 18. Indicator 500 is located at the pilot'sstation in the aircraft, often called the cockpit. That indicatorreceives a signal from a detector 505, which responds to the voltagesignal produced by capacitor C in FIG. 19, which will be termed a sparksignal. If the spark signal indicates that the igniter is producingsparks, then the indicator 500 indicates the presence of spark, as byproducing a light. If the spark signal is absent, then the indicator 500indicates the absence of spark, as by producing no light.

From another perspective, the indicator 500 operates oppositely to ananalogous indicator in an automobile. The oil-pressure indicator lightin an automobile, for example, illuminates when a problem occurs. Incontrast, the indicator 500 illuminates when a problem is absent,namely, when the igniter is producing spark.

A switch 510 can be provided, to allow the pilot to turn off theindicator 500 when knowledge of spark is not desired. Alternately, acontrol system, not shown, can control when the indicator 500 displaysits information.

FIG. 36 illustrates two forms of the invention. The igniter 550 has aproximal end 555 and a distal end 560. The proximal end has a housing565, which, in this instance, is of cylindrical shape, or circular shapein cross-section. Other cross-sectional shapes are possible.

A detachable housing 570 is shown, which contains coil L, capacitor C,and resistor R, analogous to the corresponding elements in FIG. 29, andindicated by block RLC. Connectors, wiring leads, coaxial cable 575 orthe like allow external detection of a voltage across capacitor C, orother selected component.

Housing 570 contains an aperture 580 of a cross-sectional shape whichmatches that of the housing 565. Matching shape means that the twoshapes are the same shape, and the same size, so that the aperture 580fits snugly about the housing 565.

In one form of the invention, the inductor L is wrapped about the axisof the igniter, as in FIG. 19. In another form of the invention, ahigh-permeability ring surrounds the igniter, and captures the magneticfield B produced by current entering the igniter. The inductor L iswrapped around the ring, as in FIG. 20.

In another form of the invention, the spark detector RLC is madeintegral with the igniter, as shown in igniter 600 in FIG. 36.

Several additional aspects of the invention are the following. In oneform of the invention, an adapter 311 in FIG. 19A is provided whichmounts to the engine or combustor (neither shown). The igniter threadsinto the adapter. The adapter contains integral coils 310 which performthe detection function described above.

It is not necessary that the RLC circuit operate at resonance. Rather,the RLC circuit can be viewed as performing two functions. One is thatit amplifies the pulse generated in the inductor, L. A second is thatthe RLC circuit produces ringing, or a decaying sinusoid. The ringingcauses the detected signal to persist over a longer time than the pulseinducing the ringing, thereby making the pulse easier to detect.

The triangular wave shown for example in FIG. 34 is diagrammatic. Theactual signal used in the igniter need not be triangular, but willdepend on the requirements of the particular igniter used. Also, thephysical properties of the igniter change as it ages, and thoseproperties affect the shape of the pulse applied to it. By analogy, itis well known that, in a capacitor, the current is not in phase with anapplied AC voltage. The internal resistance of the capacitor may changewith age. That change will cause a change in the phase angle between thecurrent and the voltage, thus illustrating the point that a change in aphysical object to which a voltage is applied can change the resultingcurrent in the object.

The discussion herein was framed in terms of discrete, lumped circuitelements, such as the R, L and C in FIG. 24. However, it is observedthat similar results can be obtained with distributed elements.

The application to gas turbine engines in aircraft discussed above isexemplary. In general, the invention is applicable to gas turbinesgenerally, which are used in aircraft, land vehicles, ships, powergeneration, and other applications. Further, the invention is applicableto spark detection in igniters generally.

A bleed resistor can be added to bleed charge from capacitor C in FIG.24, and the corresponding capacitor in the RLC circuit. The bleedresistor sharpens the decay of the ringing, thus causing the ringing todie out faster.

In general, the values of R, L, and C are chosen to provide a detectablesignal, such as on capacitor C. One definition of detectable signal canbe obtained with reference to K-type thermocouples, which are usedextensively in gas turbine engines. Such thermocouples produce signalsin the range of 250 millivolts. Thus, one definition of a detectablesignal can be a signal exceeding 250 millivolts. Consistent with thatdefinition, in one experiment, the inventors obtained a signal of 470millivolts across the capacitor C.

One feature of the invention is that it allows the capacitor in the RLCcircuit to be positioned remotely from the other components, and thus ina cooler location than the location of the coil 310 in FIG. 19. In oneembodiment, the capacitor C can be positioned in a room-temperatureenvironment, or cooler, where room temperature is taken as nominally 75degrees F.

This can be significant because many capacitors have a practicaltemperature limit of 175 degrees F. But the location of coil 310 in FIG.19 will probably exceed 400 degrees F.

The discussion above referred to delivering a signal indicating properspark is occurring to a pilot station. This signal could also bedelivered to maintenance personnel, either in the apparatus whichutilizes the gas turbine engine, or to remote maintenance personnel. Thesignal can also be delivered to more than one pilot station.

Numerous substitutions and modifications can be undertaken withoutdeparting from the true spirit and scope of the invention. For example,the discussion above was framed in terms of aircraft gas turbineengines. However, the invention can be used in other types of gasturbines, such as land-based gas turbine engines used in powergeneration and pumping, or in ships. In addition, the invention is notrestricted to gas turbine engines, but can be used in ignitersgenerally, which are used in various combustion applications.

Also, it is not required that the invention be operated in a hotenvironment, but the invention does provide the ability to withstandhigh-temperatures, as explained above.

What is desired to be secured by Letters Patent is the invention asdefined in the following claims.

1. A method of detecting spark in an igniter in a gas turbine engine,comprising: a) providing a transformer having i) a primary which carriesigniter current and ii) a secondary of inductance L; b) connecting thesecondary in series with a resistance R and capacitance C; and c)inferring presence of spark by detecting signals in capacitance C.
 2. Amethod of detecting spark in an igniter in a gas turbine engine,comprising: a) providing a transformer having i) a primary which carriesigniter current and ii) a secondary of inductance L; b) connecting thesecondary in series with a resistance R and capacitance C; and c)inferring presence of spark by detecting signals in capacitance C,wherein a cable connects to the igniter, the cable and the igniter aresurrounded by an conductive electrical shield connected to an engineframe, and the secondary comprises a coil wrapped around part of theshield, wherein the core of the coil comprises said part.
 3. Apparatus,comprising: a) an igniter which is effective to reliably ignite fuel ina gas turbine engine; b) a coil adjacent a housing of the igniter, whichproduces signals when sparks are generated in the igniter.
 4. Apparatusaccording to claim 3, wherein the coil comprises an inductor, andfurther comprising: c) a capacitor in series with the inductor, d) aresistor in series with the capacitor, wherein the capacitor, theresistor, and the coil form an RLC circuit which amplifies a signal inthe coil.
 5. Apparatus for detecting spark in an igniter in a gasturbine engine, comprising: a) a transformer having i) a primary whichcarries igniter current and ii) a secondary of inductance L; b) aresistance R and capacitance C in series with the inductance L; and c) adetector for inferring spark by detecting signals in capacitance C. 6.Apparatus for detecting spark in an igniter in a gas turbine engine,comprising: a) a transformer having i) a primary which carries ignitercurrent and ii) a secondary of inductance L; b) a resistance R andcapacitance C in series with the inductance L; and c) a detector forinferring spark by detecting signals in capacitance C, wherein a cableconnects to the igniter, the cable and the igniter are surrounded by anconductive electrical shield connected to an engine frame, and thesecondary comprises a coil wrapped around part of the shield, whereinthe core of the coil comprises said part.
 7. Apparatus, comprising: a) agas turbine engine having a frame or casing having a potential definedas DC ground; b) an igniter in the engine; c) a supply cable whichsupplies current pulses to the igniter; d) a conductive shield aroundthe supply cable, which connects to a housing of the igniter, whereinthe shield and the housing are connected to said ground potential; e) anexciter which provides said current to the igniter, and which receivesreturn current from the igniter through i) said shield, and ii) a secondpath; f) a detector comprising a coil and adjacent said housing, whichdetects one or more of the following: i) current pulses in the cable;ii) current pulses in the housing; or iii) differential between currentpulses in the cable and current pulses in the housing.
 8. Apparatusaccording to claim 7, wherein the coil comprises an inductance L, andfurther comprising: g) resistance R and capacitance C which, togetherwith the coil, form an RLC circuit.
 9. Apparatus according to claim 8,wherein the capacitance C in the RLC circuit produces a signal voltageexceeding 50 millivolts in response to each current pulse.
 10. Apparatusaccording to claim 3, wherein the housing is conductive.
 11. Apparatusaccording to claim 10, wherein the housing is connected to a systemground.
 12. Apparatus according to claim 3, wherein the housing acts asa grounded shield which prevents personnel from contacting high voltagesused by the igniter.
 13. Apparatus according to claim 12, wherein (1)the grounded shield surrounds a conductor, (2) the conductor carriesincoming current to a spark gap in the igniter, (3) the grounded shieldcarries return current from the spark gap, which is less than theincoming current, so that (4) magnetic fields of the incoming current donot cancel completely magnetic fields of the return current, therebyproviding a net magnetic field for the coil to detect.
 14. Methodaccording to claim 1, wherein the capacitor C produces a decayingoscillating signal in response to spark.
 15. Apparatus according toclaim 5, wherein the capacitor C produces a decaying oscillating signalin response to spark.